Invasive toads give a boost to crimson finches

A finch species in Australia is getting help from an unexpected ally: invasive cane toads that have poisoned many of the lizards that attack the birds’ nests.

Cane toads have proven to be an unstoppable scourge as they move west across Australia. Along the way, predators have snacked on the toxic toads and died.

Researchers decided to investigate the toads’ effects on not only the predators, but also the prey that those animals normally eat. The predators studied included three species of monitor lizards and the freshwater crocodile, and the prey species included the crimson finch, Gilbert’s dragon lizard, and common tree snake.

To track the toads’ invasion, the team surveyed 13-kilometer-long stretches of a road near the Ord River in western Australia from 2009-2013. One researcher drove along the road, while two passengers searched for toads using spotlights.

During the same time period, the team looked for the monitor lizards, crocodiles, Gilbert’s dragons, and common tree snakes on the shores of the Ord River and on nearby piers, docks, logs, trees, and other plants. The researchers also studied crimson finch nests in the area from 2011-2013, searching for holes and other signs that the nests had been attacked. And they monitored 12 nests with cameras to find out which predator species were eating the eggs and nestlings.

The cane toads likely invaded the area in 2010-11, the team found. Two monitor lizard species, the Mitchell’s water monitor and Mertens’ water monitor, were hit hard. The numbers of those lizards dropped by 49 and 41 percent, respectively, after the invasion.

The scientists also spotted both monitor lizard species eating finch eggs and nestlings. As those lizards declined, the finches’ fledging success rate rose from 55 to 81 percent, the researchers report.

“[I]nvasive cane toads are causing monitor lizards to lose a foothold on top-down regulation of their prey,” the authors write in Ecology. And other birds could benefit as well. The threatened purple-crowned fairy-wren lives in the Kimberley region, which will likely soon be invaded by toads. About half of the wrens’ nests are usually attacked, but the birds could fare better as the toads poison more predators. — Roberta Kwok | 12 March 2015